In order to suppress an explosion in an underground coal mine, explosion barriers are presently employed to release explosion suppression substances, such as water, into the path of the explosion. The explosion suppressing substance stops propagation of the flame.
The types of explosion barriers are generally classified as being either (a) triggered or (b) passive.
Triggered barriers typically use optical and/or electronic devices to sense the flame accompanying an explosion and set off explosives or open high pressure containers to rapidly expel the explosion suppressing substance. The flame sensor is mounted between the explosion source and the suppressant container to allow time for the suppressing agent to be discharged prior to arrival of the flame. These devices are extremely fast acting and require less suppressant material to stop an explosion than the passive barrier. While generally satisfactory, such triggered barriers are expensive and require a substantial amount of maintenance to insure their proper operation.
Passive barriers typically comprise a receptacle containing the explosion suppressing substance, such as water. The receptacle is placed on a platform or in a frame located beneath the mine roof.
In operation, the receptacle is blown off the platform or pulled out of the frame by the strong air current which precedes an explosion. The suppressant is then dispersed into the path of the explosion.
While generally satisfactory for explosions causing moderate to high wind speeds (in excess of 250 ft./sec.), passive barriers of this type are ineffective in suppressing slow moving explosions. More specifically, the slow moving wind is not capable of spilling the contents of the barrier. Accordingly, passive barriers of this type are generally located no nearer than approximately 160 ft. from the explosion source.
A prior passive explosion barrier of particular interest here is that disclosed in U.S. Pat. No. 3,960,217 (Liebman et al). The explosion barrier disclosed in that patent comprises a receptacle containing an explosion suppressing substance mounted on a pair of beams adjacent the mine roof with a pair of support flanges. The first flange is formed on the upper edge or rim of one side of the receptacle. The other flange is formed on the upper edge of a face plate hinged to the base of the receptacle on the side thereof opposite the first flange. With the barrier mounted on the beams, the face plate is angled away from the receptacle, oblique to a horizontal plane. Friction between the face plate flange and beam prevents the flange from slipping from the beam due to accidental disturbances. However, a wind generated by an explosion will act against the face plate and create forces tending to both lift the plate and pivot it against the receptacle. The lift reduces the friction between the face plate flange and beam causing the face plate to more easily pivot against the receptacle and slip from the beam in response to relatively low velocity wind.
The chief disadvantage of the barrier device of the Liebman et al patent is cost. Thus, while the cost is substantially decreased as compared with triggered barriers, the barrier device of the Liebman et al patent is relatively complicated and expensive to construct.